1. Field of the Invention
The present invention relates to a magnetoresistive element. More particularly, it relates to, for example, a magnetoresistive element in which a current can bidirectionally be supplied to record information.
2. Description of the Related Art
In recent years, a large number of memories in which information is stored in accordance with a new principle have been suggested, and as one of them, a magnetic random access memory (MRAM) is known in which the tunneling magnetoresistive (TMR) effect is utilized.
The MRAM stores binary data “1”, “0” by a magnetic tunnel junction (MTJ) element. This MTJ element has a structure in which a nonmagnetic layer (a tunnel barrier layer) is sandwiched between two magnetic layers (a free layer and a pinned layer). The information to be stored in the MTJ element is judged on whether the spinning directions of two magnetic layers are parallel or anti-parallel.
In a spin injection MRAM, a current is supplied in a direction perpendicular to the film surface of the MTJ element, and the spin is injected into the free layer in accordance with the direction in which the current is supplied, to cause magnetization reversal. When the MTJ element is of a perpendicular magnetization type, uniaxial anisotropy is imparted in the direction perpendicular to the film surface. Unlike an in-plane magnetization type, magnetic shape anisotropy does not have to be imparted in a planar direction. Therefore, the aspect ratio of the MTJ element can be set substantially to 1 to minimize the MTJ element to a processing limit in principle. Moreover, unlike the in-plane magnetization type, current magnetic field interconnects for biaxially generating current magnetic fields in different directions become unnecessary. So long as two terminals connected to upper and lower electrodes of the MTJ element are present, the MTJ element can operate, so that a cell area per bit can be reduced.
In recent years, it has been demonstrated that (001) face oriented polycrystalline magnesium oxide (MgO) as the tunnel barrier layer of the MTJ element is sandwiched between similarly (001) face oriented polycrystalline CoFeB's to allow MgO to operate as a spin filter, and electrons are supplied from the pinned layer to the free layer to reverse the magnetization of the free layer from the anti-parallel magnetization to the parallel magnetization, or conversely, the electrons are supplied from the free layer to the pinned layer to reverse the magnetization of the free layer from the parallel magnetization to the anti-parallel magnetization, and in addition, this magnesium oxide (MgO) is a promising material which realizes the spin injection MRAM having a high TMR effect (see Documents 1 and 2).
Document 1 (K. Tsunekawa et al., “Giant Magnetoresistive Tunneling Effect in Low-resistance CoFeB/MgO(001)/CoFeB Magnetic Tunnel Junctions for Read Head Applications”, Appln. Phys. Lett. 87, 072503 [2005])
Document 2 (H. Kubota et al., “Evaluation of Spin-Transfer Switching in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions”, Jpn. J. Appln. Phys. 44, pp. L1237 to L1240 [2005])
In addition, to decrease write current, the volume of the free layer, saturation magnetization Ms, a dumping constant and the like need to be decreased. However, there is a physical limit on film thinning for decreasing the volume of the free layer, and there is a processing limit on area reduction in the planar direction. Moreover, when the dumping constant is excessively lowered, thermal stability deteriorates, and hence parameters need to be adjusted to be entirely balanced, thereby decreasing the write current, and it is not easy to thin the free layer. If the write current cannot be sufficiently decreased, a power source voltage of a circuit is usually constant, and hence the film thickness of an MgO barrier needs to be decreased to lower a resistance, thereby setting a necessary write current. Therefore, the MgO barrier which is a constitutional requirement for the MTJ element needs to be a sufficiently thin film, and a high voltage stress continues to be applied during use.
The MgO barrier and CoFeB formed on and under the MgO barrier are polycrystalline, but they are laminated on the (001) face together as microscopically viewed. As to the MgO barrier, in a mainstream process at present, an MTJ laminated film is formed by a sputtering process, and then the film is annealed at a desired temperature (e.g., 360° C.) to crystallize the same. Such a method may bring about an interface defect due to lattice mismatch present between MgO and CoFeB, and various defects such as Mg defect, oxygen defect, and polycrystalline grain boundaries present in the MgO barrier owing to the use of the sputtering process. In a case where the MTJ element using the MgO barrier is used in an MRAM cell, it is feared that the existing defective portions become triggers of deterioration and hence the MgO barrier deteriorates earlier than its intrinsic life. Above all, in a metal oxide based insulator, the oxygen defect is easily generated. A countermeasure for decreasing the defect is important, but the countermeasure is not sufficient in the present mainstream process.
It is to be noted that the MTJ element in which magnesium oxide (MgO) is used as the tunnel barrier layer has been described above, but also in a case where aluminum oxide is used, the problem of the oxygen defect is basically present, and the countermeasure is necessary.